1. Field of the Invention
The present invention is directed toward a Metal-Clad type cable. More particularly, the present invention relates to a Metal-Clad type metal cable assembly which includes electrical conductors each having a conventional layer of insulation, a jacketing layer and an extruded protective layer.
2. Discussion of Related Art
Armored cable (“AC”) and Metal-Clad (“MC”) cable provide electrical wiring in various types of construction applications. The type, use and composition of these cables must satisfy certain standards as set forth, for example, in the National Electric Code (NEC®). These cables house electrical conductors within a metal armor. The metal armor may be flexible enabling the cable to bend while protecting the conductors against external damage during and after installation. The armor which houses the electrical conductors may be made from steel or aluminum. Typically, the metal armor sheath is formed from strip steel, for example, which is helically wrapped to form a series of interlocked “S” shaped sections along a longitudinal length of the cable. Alternatively, the sheaths may be made from smooth or corrugated metal.
Generally, AC and MC cable have different internal constructions and performance characteristics and are governed by different standards. For example, AC cable is manufactured to UL Standard 4 and can contain up to four (4) insulated conductors individually wrapped in a fibrous material which are cabled together in a left hand lay. Each electrical conductor is covered with a thermoplastic insulation and a jacket layer. The conductors are disposed within a metal armor or sheath. If a grounding conductor is employed, the grounding conductor is either (i) separately covered or wrapped with the fibrous material before being cabled with the thermoplastic insulated conductors; or (ii) enclosed in the fibrous material together with the insulated conductors for thermoset insulated conductors. In either configuration, the bare grounding conductor is prevented from contacting the metal armor by the fibrous material. Additionally in type AC cable, a bonding strip or wire is laid lengthwise longitudinally along the cabled conductors and the assembly is fed into an armoring machine process. The bonding strip is in intimate contact with the metal armor or sheath providing a low-impedance fault return path to safely conduct fault current. The bonding wire is unique to AC cable and allows the outer metal armor in conjunction with the bonding strip to provide a low impedance equipment grounding path.
In contrast, MC cable is manufactured according to UL standard 1569 and includes a conductor assembly with no limit on the number of electrical conductors having a particular AWG (American Wire Gauge). The conductor assembly may contain a grounding conductor. The electrical conductors and the ground conductor are cabled together in a left or right hand lay and encased collectively in an overall covering. Similar to AC cable, the assembly is then fed into an armoring machine where metal tape is helically applied around the assembly to form a metal sheath. The metallic sheath of continuous or corrugated type MC cable may be used as an equipment grounding conductor if the ohmic resistance satisfies the requirements of UL 1569. A grounding conductor may be included which, in combination with the metallic sheath, would satisfy the UL ohmic resistance requirement. In this case, the metallic sheath and the grounding/bonding conductor would comprise what is referred to as a metallic sheath assembly.
As mentioned above, prior AC cables include a fibrous cover over each of the electrical conductors and if a grounding conductor is used, the fibrous material is disposed between the grounding conductor and the metal armored sheath. MC cable includes either a covering over all of the electrically insulated conductors and the grounding conductor after cabling or a covering over just the electrical insulated conductors combined after cabling while the grounding conductor is positioned externally separate from this overall covering. This covering material is typically a nonmetallic material composed of polypropylene or polyester. However, this covering material does not provide conductor to conductor mechanical protection nor does it provide protection within an enclosure such as a junction box or panel when the cable is installed therein. Thus, there is a need for an improved MC armored cable that provides added mechanical protection to the conductors housed within an electrical cable assembly.